1. Field of the Invention
Aspects of the present invention relate to a fixing apparatus of an image forming device. More particularly, aspects of the present invention relate to a heat roller for a fixing apparatus, having a coupling structure capable of preventing slip of a roller cap while the heat roller is driven.
2. Description of the Related Art
Generally, an image forming device, such as a laser printer or a copier, forms an image by printing out a desired image through a series of processes including electrification, exposure, development, transfer and fixing. The fixing process fixes a transferred toner image on a print medium, such as a paper sheet, a transparency, etc., by applying heat and pressure to the toner image, and is performed by using a pair of rollers facing each other.
The pair of rollers is a press roller and a heat roller. FIG. 1 illustrates the structure of a heat roller of the related art. Referring to FIG. 1, the heat roller 10 has a conductive cylindrical roller pipe 11, a resistive heat element 12 accommodated in the conductive cylindrical roller pipe 11, and caps 13 and 14 coupled to opposite ends of the conductive cylindrical roller pipe 11. A coating layer 18a is formed on the outer surface of the conductive cylindrical roller pipe 11. The resistive heating element 12 is generally an induction coil, and an insulation layer 18c may be interposed between the conductive cylindrical roller pipe 11 and the resistive heat element 12. The heat roller 10 also has an internal expanding pipe 16 which strongly pushes the resistive heat element 12 toward the roller pipe 11. The caps 13 and 14 are an end cap 13 and a gear cap 14, respectively. Each of these caps 13 and 14 has a connection terminal 15 and a Cu—Sn electrode 13a or 14a. 
In the heat roller 10 structured as described above, current flows through the resistive heat element 12 via the electrodes 13a and 14a and the connection terminals 15 and 15 when alternative current (AC) from an AC power source is applied to carbon brushes 17a and 17b provided at opposite ends of the heat roller 10. As a result, energy which is generated due to resistant heating caused by the resistive heat element 12 is transferred to the roller pipe 11, which is an object to be heated, through the insulation layer 18b, and is further transferred to toner and the print medium through the roller pipe 11 and the coating layer 18a. 
In the typical heat roller 10, the caps 13 and 14, particularly the gear cap 14, are released from the heat roller pipe 11 if the caps 13 and 14 are not securely coupled to the ends of the heat roller pipe 11, which results in twisting of the connection terminals 15, and can cause a failure, such as a short circuit. Accordingly, the caps 13 and 14 need to be securely fixed to the ends of the heat roller pipe 11.
FIG. 2 is an exploded perspective view of the gear cap 14 of the heat roller shown in FIG. 1. Referring to FIG. 2, the gear cap 14 has male screw threads 14b which can be securely mated with female screw threads 11b formed on the inner surface of the heat roller pipe 11. Even though the end cap 13 is not shown in FIG. 2, the end cap 13 is also screw mated with the heat roller pipe 11.
However, in the heat roller for a fixing apparatus as described above, even if the gear cap 14 is coupled with the heat roller pipe 11 by a screw coupling manner, the gear cap 14 can slip off the heat roller pipe 11 during counter rotation. When the gear cap 14 slips off the heat roller pipe 11, the connection terminals 15 and 15 for connecting the electrodes 13a and 14a with the resistive heat element 12 become twisted, causing a short circuit. Additionally, the heat roller pipe has a diameter of about 1 mm so that height of the female screw threads formed on the inner surface of the heat roller pipe is about 0.35 mm at most. Accordingly, if torque is too high, the screw threads are demolished, resulting in rotating of the gear cap. This rotating can cause a variety of problems, including a short circuit, contact failure, sparking, and so on.
In order to solve the above described and/or other problems, the applicant of the present invention suggested a heat roller for a fixing apparatus, in which a gear cap does not have screw threads but is coupled to a heat roller pipe by a different coupling device other than the screw configuration. Such a heat roller is disclosed in Korean Patent Registration No. 452,874.
FIG. 3 and FIG. 4 illustrate the structure of the coupling device of the gear cap and the heat roller pipe disclosed in the above literature. Particularly, FIG. 3 is an exploded perspective view and FIG. 4 is a sectional view. Referring to FIG. 3, a gear cap 24 is coupled to an end portion of a heat roller pipe 21 by a fixing ring 30 and a latch projection 32a. The fixing ring 30 restricts the gear cap 24 from slipping off the heat roller pipe 21 in the axial direction of the heat roller pipe 21, and the latch projection 32a received in an engagement groove 31a restricts the gear cap 24, which is restricted in the axial direction, from slipping the heat roller pipe 21 in the rotational direction.
The above described coupling structure of the gear cap 24 has a greater coupling force than the screw coupling configuration. However, in the situation when the heat roller 20 is applied to a high speed printer, the following problem occurs. When inertial force of the heat roller 20 is greater than the expanding pressure of an internal expanding pipe 26, the internal expanding pipe 26 slips in the rotational direction, thereby causing the resistive heat element 22 and an insulation layer 28b to slip off, and causing a short circuit after the heat roller 20 repeatedly performs the operations of rotating and stopping at high speed. The internal expanding pipe 26 of the above described heat roller 20 has expansivity higher than that of an insulative mica. Accordingly, an image forming device including a heat roller such as heat roller 20 repeatedly performs the operations of heating and cooling corresponding to the times when a user turns the image forming device on and off. In this instance, since the cap 24 is coupled only by the fixing ring 30, the internal expanding pipe 26 can slip, thereby causing a short circuit of an induction coil, which is the resistive heat element 22.